Magnetic-field-sensing field-effect transistor

ABSTRACT

A MEGNETIC-FIELD-SENSING DEVICE IS CONSTRUCTED BY SUSPENDING AN ELECTRICALLY CONDUCTIVE BAR,THAT HAS AT LEAST ONE MAGNETIC REGION, ABOVE THE CHANNEL BETWEEN THE SOURCE AND THE DRAIN OF A METAL-OXIDE-SEMICONDUCTOR FIELD-EFFECT TRANSISTOR (MOS). THE SUSPENDED BAR ACTS AS THE GATE ELECTRODE AND IS CONNECTED TO A D.C. BIAS VOLTAGE SUPPLY. THE SUSPENDED BAR CAN BE MADE BY OVERLAYING MAGNETIC MATERIAL ON A SUITABLE ELECTRICAL CONDUCTOR, OR IT CAN BE CONSTRUCTED ENTIRELY OF AN E LECTRICALLY CONDUCTIVE MAGNETIC MATERIAL, WHICH MAY BE A HIGH-RESISTANCE MATERIAL IF DESIRED. A MAGNETIC FIELD IN THE VICINITY OF THE SUSPENDED BAR CAUSES MECHANICAL DISPLACEMENT OF THE SUSPENDED BAR, THEREBY ALTERING THE ELECTROSTATIC FIELD IN THE VICINITY OF THE CHANNEL REGION. THE ALTERED ELECTROSTATIC FIELD AFFECTS THE INVERSION LAYER WITHIN THE CHANNEL, CAUSING A CHANGE IN CONDUCTIVITY, THUS PRODUCING A CHANGE IN THE SOURCE-TO-DRAIN CURRENT AS A FUNCTION OF THE MAGNETIC FIELD.

1971 w.|- PUTERBAUGH, JR 5 MAGNETIC-FI ELD-$ENSING FIELD-EFFECT TRANSISTCVR Original Filed March 13, 1967 FIG.-l

INVENTOR ms ATTORNEYS WILLIAM H. PUTERBAUGH, JR.

United States Patent 3,553,540 MAGNETIC-FIELD-SENSIN G FIELD-EFFECT TRANSISTOR William H. Puterbaugh, Jr., Sunnyvale, Calif., assignor to The National Cash Register Company, Dayton, Ohio, a corporation of Maryland Continuation of application Ser. No. 622,742, Mar. 13, 1967. This application Jan. 8, 1969, Ser. No. 789,756

Int. Cl. H01] 11/14 U.S. Cl. 317-235 9 Claims ABSTRACT OF THE DISCLOSURE A megnetic-field-sensing device is constructed by suspending an electrically conductive bar, that has at least one magnetic region, above the channel between the source and the drain of a metal-oxide-semiconductor field-effect transistor (MOS). The suspended bar acts as the gate electrode and is connected to a DC bias voltage supply. The suspended bar can be made by overlaying magnetic material on a suitable electrical conductor, or it can be constructed entirely of an electrically conductive magnetic material, which may be a high-resistance material if desired. A magnetic field in the vicinity of the suspended bar causes mechanical displacement of the suspended bar, thereby altering the electrostatic field in the vicinity of the channel region. The altered electrostatic field atfects the inversion layer within the channel, causing a change in conductivity, thus producing a change in the source-to-drain current as a function of the magnetic field.

CROSS REFERENCE TO RELATED APPLICATION This application is a continuation of United States patent application Ser. No. 622,742, filed Mar. 13, 1967, now abandoned in the name of William H. Puterbaugh, Jr., inventor.

BACKGROUND OF THE INVENTION The present invention provides a magnetic-field-sensin-g metal-oxide-semiconductor 'field-efiect transistor. A metal-oxide-semiconductor field-effect transistor with an electrically conductive bar suspended above the channel between the source and the drain is described in Electronics, Sept. 20, 1965, pages 84 to 87, by Harvey C. Nathanson, William E. Newell, and Robert A. Wick strorn. The electrically conductive bar in the Nathanson et al. device resonates at its fundamental mechanical frequency in response to an electrical input signal that is coupled to a fixed metal plate which acts as one electrode of a capacitor that is spaced apart from the end of the conductive bar, which acts as the other electrode of the capacitor. The Nathanson et al. device relies on capacitive coupling between the fixed capacitor plate and the conductive bar to sustain a resonant vibration. of the conductive bar. The Nathanson et al. device, moreover, is employed as a frequency selective filter device and not as a magnetic-field-sensing device.

The present invention differs from the Nathanson et al. device in that the suspended bar, which is both magnetic and electrically conductive, is mechanically displaced whenever it is placed in the vicinity of a magnetic field. Although the suspended bar of the present invention undergoes a mechanical displacement in response to the magnetic field, mechanical resonance of the suspended bar is not required nor normally desired.

SUMMARY OF 'UHE INVENTION A field-effect transistor that has a gate electrode with a deflectable portion which is suspended over the channel region between the source and the drain of the field-effect 3,553,540 Patented Jan. 5, 1971 transistor and which has at least one ferromagnetic region, the suspended portion of the gate electrode, therefore, being deflected when it -is in the vicinity of a magnetic field to effect a change in the source-to-drain current of the field-effect transistor as a function of the magnetic field. The preferred embodiment of the invention is a metal-oxide-serniconductor transistor.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a top view of the field-effect transistor of the present invention.

.FIG. 2 is a side sectional view of the fieldeffect transistor of the present invention.

FIG. 3 is a top view of another embodiment of the field-effect transistor of the present invention.

FIG. 4 is a side sectional view of a memory device embodiment of the present invention.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT A layer of silicon oxide 12 of FIG. 2 overlies the channel 16. The channel 16 becomes conductive by the formation of an inversion layer when the suspended bar 22, acting as a gate electrode, is connected to the proper bias voltage. The metal-oxide-semiconductor field-effect transistor operates in the enhancement mode, with N-type (diffused) source and drain regions in a P-type substrate, and, hence, the charge carriers are electrons. The enhancement mode of operation occurs when no current is conducted unless the gate is biased with the same polarity as the drain electrode. The gate electrode in FIGS. 1 and 2 is a suspended conductive and magnetic bar 22. The conductive and magnetic bar 22 may be secured at both ends, or it may be of cantilever construction and secured at anly one end. Since the suspended bar 22 is insulated from the source 28 and the drain 30, the input resistance of the present device is very high, on the order of 10* ohms, at low frequencies.

The charge carriers in the inversion layer of a metaloxide-semiconductor field-effect transistor may be modulated by varying either the gate voltage or the gate spacing. In the present invention, the gate spacing is varied, and the bias voltage is held constant. The modulation of the electrostatic field of the present invention is determined by the deflection of the suspended bar 22, which acts as the gate electrode, The deflection of the bar 22 is in turn determined by the magnetic field which is coupled to the magnetic region 24. The magnetic region 24 may be formed of any magnetic material that can be deposited on or secured to the bar 22. The magnetic material may have either a linear or a rectangular hysteresis characteristic, and it may be a thin film deposit.

The operating point of the field-effect transistor is adjusted by applying operating potentials, in the manner known in the art, so as to bias the operating point of the field-effect transistor near, approximately, the center of the linear region of its operating characteristic when the suspended bar 22 is not displaced. The source 28 is coupled to a fixed potentialfor example, ground-and the drain 30 is coupled, for example, to a positive potential to achieve the proper biasing conditions. Movement of the suspended bar 22 away from the channel 16 causes a reduction of the source-to-drain current, and movement of the suspended bar 22 towards the channel 16 causes an increase in the source-to-drain current. Information contained in the associated magnetic field may, therefore, be detected through modulation of the source-to-drain current of the field-effect transistor of the present invention.

The preferred embodiment of the present invention may be manufactured by modification of the manufacturing steps that are employed to produce a conventional insulated-gate metal-oxide-semiconductor field-effect device. The device of the preferred embodiment may be processed by conventional steps up to the point where the source 28, the drain 30, and the associated electrodes 20 have been formed, and the silicon oxide layer 12 has been thermally grown on the semiconductor body 10. The source and the drain electrodes 20 may be interchangeably employed as in most conventional metal-oxide-semiconductor field-effect transistors. It is preferred that the source 28 and the drain 30 be P regions in an N type semiconductor body 10. The silicon oxide layer 12 is initially at a uniform level across the width of the semiconductor body 10, except where the source and drain electrode connections 20 are to be made. The gate electrode, however, does not rest on the silicon oxide layer 12, as in a conventional insulated gate metal-oxide-semiconductor field-effect transistor.

Instead, after etching a portion of the silicon oxide layer over the channel 16, so that it is thinner than the remaining portion of the silicon oxide layer 12, a photoresist is applied over the entire upper surface of the semiconductor body 10. Windows are then etched for the source and drain connections 20, and gold is deposited over the upper surface of the semiconductor body by sputtering. A magnetic layer is then deposited over a portion of the gold layer to cover the bar 22. A photoresist is selectively applied, and the bar 22 and the source and drain areas 28 and 30 are exposed through a mask, so that gold is removed everywhere except for the bar 22 and the source and drain connection 20, and the magnetic overcoating layer is removed everywhere, except for the bar 22, by etching. After removal of the unwanted gold and magnetic material, the remaining photoresist is removed, thus leaving the bar 22 suspended over the thin silicon oxide layer over the channel 16. The magnetic material 24 may completely or only partially surround the conductive bar 22. If the magnetic material 24 completely surrounds the conductive bar 22, a small electrode area must remain free of magnetic material to allow an external bias voltage connection to be made to the conductive bar 22.

The embodiment of FIG, 1 is useful in applications where it is only necessary to sense the presence of a magnetic field and the polarity of the sensed signal is not important. If it is necessary to sense the polarity of the magnetic field, the embodiment of FIG. 3 may be employed. The embodiment of FIG. 3 shows two magnetic areas, 42 and 44, that are disposed linearly along the suspended bar 22. The two magnetic areas 42 and 44 are spaced apart, with like polarity ends facing each other. The magnetic areas 42 and 44 may be as small as 0.001 inch wide, with a gap as small as 0.0001 inch. Thus, if two north magnetic poles 46 and 48 are facing each other, as in FIG. 3, a south pole data bit under the north magnetic poles 46 and 48 will cause attraction of the poles 46 and 48 towards the data bit, resulting in a deflection of the suspended bar 22 way from the channel 16, thus decreasing the source-to-drain current flow in the metal-oxidesemiconductor field-effect transistor. If an opposite polarity magnetic data bit-i.e., a north magnetic pole data bit-is under the north magnetic poles 46 and 48, a repulsive force occurs, and the suspended bar 22 is deflected towards the channel 16, thus increasing the sourceto-drain current flow in. the metal-dxide-semiconductor 4 field-effect transistor. The field-effect transistor in the embodiment of FIG. 3 is, therefore, able to distinguish between opposite polarity magnetic data bits.

An embodiment similar to the embodiment of FIG. 3 may be formed by severing the bar 22 between the magnetic areas 42 and 44. This embodiment is able to sense magnetic fields of low magnetic strength, since the two sections of the bar 22 will be free to move independently, and, therefore, a more sensitive device is produced.

FIG. 3 shows the placement of the typical metal-oxide semiconductor magnetic-field-sensing field-effect transistor of the present invention relative to movement of a magnetic medium containing a number of magnetic data bits, represented schematically by the dotted line 63, which pass under the magnetic poles 46 and 48. It is anticipated that magnetic tracks of extremely small dimensions can be sensed by the device of the present invention. For example, sensing of magnetic tracks having a density greater than tracks per inch is expected with the device of the present invention.

It is apparent that a device such as that shown in FIG. 3 may serve as a memory element as well as a sensing device. If both magnetic areas-for example, the areas 56 and 58 of FIG. 4, which are deposited on the suspended bar 64-are so set that their opposing faces are north poles, a 1 may be stored. If both of the magnetic areas 56 and 58 are reversed, so that their opposing faces are south poles, a 0 may be stored. Electrical currents of equal magnitudes passing through the conductors 60 and 62 in opposite directions will then establish like polarity magnetic poles at the adjacent faces of the magnetic areas 56 and 58. If equal-magnitude read currents are now passed through the conductors 60 and 6 2 in the same direction, or if a current is passed through only one or the other of the conductors 60 and 62, a deflection of the suspended bar 64 that is related to the storage state of the magnetic memory areas 56 and 58 will occur. When the bar 64 is deflected, the current flowing in the metal-oxide-semiconductor field-effect transistor is mod-* ulated, so as to indicate the storage states of the magnetic memory areas 56 and 58.

What is claimed is:

1. A magneticfield-sensing field-effect transistor comprising:

(a) a source, and

(b) a drain, and

(c) a channel region of semiconductive material between the source and the drain, and

(d) a gate electrode that has a deflectable portion which is suspended over the channel region, the deflectable portion of the gate electrode having at least one ferromagnetic region, the deflectable portion of the gate electrode being deflected when it is in the vicinity of a magnetic field to effect change in the source-to-drain current of the field-effect transistor as a function of the magnetic field.

2. A field-effect transistor as in claim 1 wherein an electrical insulating layer overlays at least a portion of the channel region.

3. A field-effect transistor as in claim 2 wherein the transistor is a 'metal-oxide-semiconductor field-effect transistor.

4. A device as in claim 1 wherein two elongated ferromagnetic regions are arranged in a spaced-apart relationship along the deflectable portion of the gate electrode, adjacent magnetic poles of the ferromagnetic regions being of the same magnetic polarity.

5. A field-effect transistor as in claim 4 wherein an electrical insulating layer overlays at least a portion of the channel region.

6. A field-effect transistor as in claim 5 wherein the transistor is a metal-oxide-semiconductor field-effect transistor.

7. A device as in claim 4 wherein the deflectable portion of the gate electrode is severed between the ferromagnetic regions.

6 8. A field-effect transistor as in claim 7 wherein an OTHER REFERENCES electrical insulating layer overlays at least a portion of Germano et al IB M Technical Disclosure Bulletin the channel region.

9. A field-effect transistor as in claim 8 wherein the August 1965 359*360 felled transistor is a metal-oxide-semiconductor field-effect tran- 5 JOHN HUCKERT, Primary Examiner sistor.

M. H. EDLOW, Assistant Examiner References Cited UNITED STATES PATENTS 3,413,573 11/1968 Nathanson 332 31 10 34 174;3734 

